You need to consider that pretty much each type of battery has a different way of detecting "full" and "low".
Since this is pretty clearly homework, I assume you are either told what sort of battery, or can choose for yourself.
If I were you, I'd pick lead acid or LiFePO4. Both have well understood and easily implemented measures of charge level.
LiFePO4 has an advantage over other Lithium technology that if you make a bad mistake you tend to get smoke rather than flame.
Once you've decided on your battery technology, you'll have to do some research into how these are charged. I assume you don't actually have to charge them, but just detect the charge level. This is pretty easy for the cells I have mentioned.
Note that temperature can affect things (especially for lead acid) but for the purposes of your homework you might be able to note, then ignore the effect, assuming that the ambient temperature is constant and reflects the battery temperature.